Wellbores are usually drilled over-balanced, where the pressure of the wellbore fluid is maintained to be greater than the pore pressure in the formations being drilled. Drilling over balanced may be useful to limit the amount of hydrocarbon flowing from formations into the well, and therefore may limit the risk of a well “blowout” and/or exposure to (toxic) formation gases at the wellsite. When drilling over-balanced, wellbore fluid (such as drilling mud filtrate) gradually seeps into the formation pore space in the proximity of the wellbore. As wellbore fluid invades the formation, a mud cake may be formed at the wellbore wall, and the filtration process may gradually diminish. It should be appreciated that the mud cake buildup and/or the invasion process(es) may take hours, and even days, depending on the formation and the constitution of the mud.
When a downhole tool is disposed in a wellbore (for example, to take a sample, to perform a drill stem test, etc.), a probe, a packer, a portion of the downhole tool body, or a combination thereof is usually pressed against the wellbore wall. One or more of these components may compact the mud cake and create a progressively sealed surface between the wellbore and the formation. On the side in contact with the formation wall, the sealed surface may be exposed to a pressure level substantially lower than the wellbore pressure level (typically close to the formation pressure level). Thereby, the sealed surface may be subjected to a net force urging the downhole tool against the wellbore wall. The net force usually increases in magnitude when the time on station increases. In some cases, the net force may prevent further movement of the downhole tool in the wellbore, resulting in expensive fishing operations, or abandoning the well. This problem, well known in the art, is sometimes referred to as differential sticking.
It should be appreciated that logging while drilling tools may be more prone to differential sticking than wireline tools, in particular because the mud cake may be still forming at the time logging while drilling tools are operated. Also, it should be appreciated that sampling tools, testing tools, or more generally downhole tools sometimes referred to as station tools, are more prone to differential sticking, in particular because these tools usually perform measurements over an extended period of time (such as 20 minutes or more) at essentially the same location in the wellbore.